The present invention relates to a fuel assembly for a boiling water reactor which is referred to as a reactor hereinafter in this specification unless otherwise stated, and a core of a reactor loaded with the above mentioned fuel assemblies.
One of the objects of the present invention is to provide an economically improved fuel assembly with a high degree of burnup and a core of the reactor. In order to attain the economical improvement of a nuclear power plant, it is effective to improve the coefficient of utilization of the plant by increasing the operation period while reducing the fuel cycle cost by remarkably increasing the degree of burnup of the fuel.
In order to increase the degree of burnup, various methods for enrichment have been devised and demonstrated, but the neutron spectrum is hardened to result in the following phenomena:
(1) the increase of the absolute value of the void coefficient; PA0 (2) the increase of the degree of reaction in the core when the temperature is low; and PA0 (3) the decrease of the control capability of burnable poisons such as gadolinium. PA0 (1) the positive utilization of the output peaking, PA0 (2) the reduction of the remaining quantity of gadolinium and PA0 (3) the improvement of the ratio between water and uranium. PA0 1 the absolute value of the void coefficient is increased, PA0 2 the degree of the reaction at the core at low temperatures is increased and PA0 3 the effect of the decrease in reaction control capability of burnable poisons such as gadolinium or the like is cancelled so that the degradation of the thermal margin and the shut-down margin can be prevented.
It follows therefore that there are fears that not only the thermal margin but also the shut-down margin are decreased.
In the case of attaining a high degree of fuel burnup by only increasing the enrichment, the cost of natural uranium for production of the fuel and the like are increased so that the degree of reduction of the costs of uranium enrichment and so on is increased. As a result, the range of the decrease in cost of the fuel cycle is increased by increasing the degree of burnup.
In view of the above, the present invention provides a fuel with a highly economical specific burnup which can substantially solve the problems associated with the performance of the core and can attain a high degree of burnup by decreasing the enrichment to as low as possible.